Cable with low structural elongation

ABSTRACT

A cable ( 211 ) is provided comprising a steel cord ( 212 ) and a polymer material ( 215 ). The steel filaments ( 213 ) of the steel cord ( 212 ) are coated with an adhesive before the penetration of the polymer material ( 215 ). The cable ( 211 ) has a structural elongation less than 0.025% and an E module 4% greater than the E module of the steel cord ( 212 ). These two improvements further decrease the total elongation of the cable at certain load.

FIELD OF THE INVENTION

The present invention relates to a cable. More specifically present invention relates to a cable with limited elongation.

BACKGROUND OF THE INVENTION

Cables and more specifically control cables are widely used to transmit movement, such as cable for window elevator system, cables used to open and close braks of scooters, bicycles and other vehicles. In these and many other applications, a limited elongation of the cable is required.

Cables are also widely used as tension member to reinforce polymer materials, such as steel cords to reinforce radial tires, cables to reinforce transmission belts, timing belts or flat hoisting belts. In these applications, a limited elongation of the cable is also required.

Generally, the tensile curve of a cable of prior art takes the form of “hockey stick” curve as illustrated by FIG. 1. At the initial elongation period, the elongation of the cable is large while the tension is low, and the curve is relative flat. At the terminal elongation period, the elongation of cable is almost linear to the tension of the cable as illustrated by line 120 in FIG. 1. The elongation of the cable increases steadily with the increase of the tension. At this stage, the increase of the elongation of the cable is proportional to the increase of the tension of the cable, i.e. Δε=Δδ/E, wherein Δδ is the increase of the elongation of the cable, Δε the increase of the tension of the cable, and E the module of the cable. This is called the elastic elongation. If we extend line 120 to intersect with the abscissa axis, the intersection point ε0 represents the structural elongation at low tensile stresses of the cable. Therefore, the elongation of a cable at certain tensile stress can be expressed by ε=ε0+δ/E. From this equation, we can see that the total elongation of a cable at certain tensile stress comprises two portions of elongations: structural elongation and the elastic elongation.

Therefore, there are two approaches to get a cable with limited elongation. One way is to decrease the structural elongation of the cable, and the other way to increase the E module of the cable, because the elongation under tensile decreases when the E module of the cable increases.

Besides, structural elongation ε0 of a cable is unstable and unpredictable, because there are a lot of facts, such as the structure of the cable, the voids between the filaments of the cable, and the pre-tension of the filaments when cabling the cable, that determine the structural elongation of a cable. Therefore, the unstable and unpredictable behavior of structural elongation of a cable causes problems to predict the total elongation of the cable under tension and more scrap during the start-up of machine producing for instance the high-precision timing belts. Hence, further reducing or almost eliminating the structural elongation ε0 can improve the predictability of the tension-elongation curve of the cable and facilitate the manufacturing process.

WO03044267A1 disclosed a cable with limited elongation, less than 0.05% at a permanent force of 50N, after being subjected to a force of 450N. This improvement is achieved by a cord comprising a steel cord and a polymer material. The low elongation is highly related to the penetration of polymer material into the steel cord, but there are limits on the penetration rate and the pressure for the extrusion process.

WO2005043003A1 disclosed a fine steel cord with low structural elongation. The low structural elongation is achieved by using a special cabling process. This special cabling process not only decreases the productivity of the cabling process but also ask for a lengthy fine tune procedure to set the tension of the filaments or strands.

SUMMARY OF THE INVENTION

It is an objective of the present invention to eliminate the drawback of the prior art. It is also an objective of the present invention to further limit the elongation of a cable without complicating the manufacturing process of the cabling process.

According to the present invention, a cable is provided comprising a steel cord and a polymer material. The steel filaments of the steel cord are coated with an adhesive before the penetration of the polymer material. The cable has a structural elongation less than 0.025%. Besides, the E module of the cable increases by more than 4% compared with the E module of the bare steel cord. These two improvements further decrease the total elongation of the cable at certain load.

The cable as subject of the invention comprises a steel cord, which on its turn comprises several steel filaments.

The tensile strength of the steel filaments for the steel cord are preferably more than 1700N/mm², or more than 2200N/mm² or even more than 2600N/mm², most preferably more than 3000N/mm² or even more than 4000N/mm². The diameter of the filaments is less than 400 μm, preferably less than 210 μm, most preferably less than 110 μm.

All filaments may have an identical diameter. Possible the diameter of the filaments may differ from each other. Preferably, the diameter of the filaments providing an inner strand of the cable is larger than the diameter of the filaments used to provide the outer strands or layer of filaments to the cable, which improves the penetration of the polymer material into the void spaces of the cable.

Steel cords have an inner layer or core, which is preferably a strand of several steel filaments. Around such core, at least one layer of additional steel elements is provided. The steel elements of the additional layer can either be steel filaments or steel strands, on its turn comprising steel filaments. Various steel cord construction may be used.

Examples here are:

-   -   Multi-strand steel cord e.g. of the m×n type, i.e. steel cords,         comprising m strands with each n steel filaments, such as         4×7×0.10, 7×7×0.18, 8×7×0.18 or 3×3×0.18; the last number is the         diameter of the steel filament expressed in mm;     -   Multi-strand steel cord, comprising a core strand of c metal         filaments, and m strands of n steel filaments, surrounding the         core strand. These steel cords are hereafter referred to as         c+m×n type cords, such as 19+9×7 or 19+8×7 cords;     -   Warrington-type steel cords;     -   Compact cords, e.g. of the 1×n type, i.e. steel cords comprising         n steel filaments, n being greater than 8, twisted in only one         direction with one single step to a compact cross-section, such         as 1×9×0.18; the last number is the diameter of the filament         expressed in mm;     -   Layered steel cord e.g. of the c+m(+n) type, i.e. steel cord         with a core of c filaments, surrounded by a layer of m         filaments, and possibly also surrounded by another layer of n         filaments, such as 2+4×0.18; the last number is the diameter of         the filaments expressed in mm.

The steel composition of the steel cord is preferably a plain carbon steel composition, i.e. it generally comprises a minimum carbon content of 0.40% (e.g. at least 0.60% or at least 0.80%, with a maximum of 1.1%), a manganese content ranging from 0.10 to 0.90% and a silicon content ranging from 0.10 to 0.90%; the sulfur and phosphorous contents are each preferably kept below 0.03%; additional micro-alloying elements such as chromium (up to 0.2 to 0.4%), boron, cobalt, nickel, vanadium . . . may be added to the composition; stainless steel compositions are, however, not excluded. The production of the steel filaments and the steel cords is performed according to known prior art techniques of wet drawing followed by cabling or bunching.

After an optional cleaning operation, the steel cord is then coated with an adhesive selected from organo functional silanes, organo functional titanates and organo functional zirconates which are known in the art for the improvement of adhesion between the steel cord and polymer material. Preferably, but not exclusively, the organo functional silanes are selected from the compounds of the following formula:

Y—(CH₂)_(N)—SiX₃

Wherein:

Y represents an organo functional group selected from —NH₂, CH₂═CH—, CH₂═C(CH₃)COO—, 2,3-epoxypropoxy, HS— and, Cl—

X represents a silicon functional group selected from —OR, —OC(═O)R′, —Cl wherein R and R′ are independently selected from C1 to C4 alkyl, preferably —CH3, and —C2H5; and

n is an integer between 0 and 10, preferably from 0 to 3.

Besides the organo functional silanes mentioned above, there are other steel PU adhesives commercially available on the market. They are sold under the name Chemosil (made by the German company Henkel) and Chemlock (made by Lord Corporation).

The polymer material used for the present invention can be any elastomeric material that can conveniently be applied to the steel cord with sufficient adhesion. More preferably a thermoplastic elastomer (TPE) can be used. Non-delimiting examples are polystyrene/elastomer block copolymers, polyurethane (PU) or polyurethane copolymers, polyamide/elastomer block copolymers, thermoplastic vulcanizates. Preferably thermoplastic polyurethane is used. Homopolymers of ester, ether or carbonate polyurethane may be used, as well as copolymers or polymer blends. Preferably, the polymer material has a shore hardness varying between 30 A and 90 D.

The polymer penetration rate of a cable as subject matter of the present invention is more than 70%, and preferably more than 90%. The steel cord used to provide a cable as subject matter of the present invention comprises several steel filaments being transformed into a steel cord, using a steel cord construction. Due to the steel cord construction, void spaces are provided between the steel filaments of the steel elements of the cord. Also void spaces are provided between the steel elements. “Void space” as used hereafter is to be understood as all area of a radial cross-section of the cord, located inwards of the imaginary circle which encircles a radial cross section of the steel cord which area is not occupied by steel. Therefore, the polymer penetration rate of a cable of present invention is defined as the ratio in percentage of the void space filled by polymer to the void space which is not occupied by steel.

The thickness of the polymer coating of the present invention is less than 100 μm, and preferably less than 10 μm. The optical diameter of the steel cord used to provide a cable as subject matter of the present invention is the diameter of the smallest imaginary circle, which encircles a radial cross section of the steel cord. The optical diameter of the cable of the present invention is the diameter of the smallest imaginary circle, which encircles a radial cross section of the cable. Therefore, the thickness of the polymer is defined as the half of the difference of the optical diameter between the cable and the steel cord.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described into more detail with reference to the accompanying drawings wherein

-   -   FIG. 1 is a tensile curve of a cable of prior arts;     -   FIG. 2 is a cross-sectional view of a cable incorporating the         present invention;     -   FIG. 3 is a tensile curve of a cable incorporating the present         invention.

-   Item 110 is the tensile curve of a cable of prior arts; -   Item 120 is a line representing the E module of a cable of prior     arts; -   Item 211 is a cable incorporating the present invention; -   Item 212 is a steel cord for the cable incorporating the present     invention; -   Item 213 is a steel filament for the cable incorporating the present     invention; -   Item 214 is the optical diameter of the steel cord for the cable     incorporating the present invention; -   Item 215 is a polymer material used for the cable incorporating the     present invention; -   Item 216 is the optical diameter of the cable incorporating the     present invention; -   Item 217 is the thickness of the polymer coating of the cable     incorporating the present invention; -   Item 218 is the void space in the cable incorporating the present     invention; -   Item 219 is the steel strand for the cable incorporating the present     invention; -   Item 310 is the tensile curve of a cable incorporating the present     invention; -   Item 320 is a line representing the E module of a cable     incorporating the present invention; -   ε0 is the structural elongation of a cable; -   F(N) is the force in Newton on the test specimen; -   E(%) is the elongation in percentage of the test specimen.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, there is shown a tensile curve 110 of a cable of prior art, and line 120 represents the E module of the cable. Line 120 extends and intersects with the abscissa axis at the intersection point ε0, representing the structural elongation at low tensile stresses of the cable.

As shown in FIG. 2, there is a cross section view of a cable incorporating the present invention. The cable 211 comprises a steel cord 212, which on its turn comprises several steel filaments 213. The present embodiment shows a steel cord with “7×7×d” structure having seven strands 219, each strand having seven steel filaments 213 of diameter of d mm. The steel cord has an optical cord diameter 214. The steel cord is coated with a polymer material 215, so providing a cable 211 as subject matter of the present invention with an optical cable diameter 216. The thickness 217 of the polymer coating is half of the difference between optical cord diameter and optical cable diameter. As shown in FIG. 2, preferably the void space 218 between the different steel filaments 213 is substantially filled with polymer material 215.

In FIG. 3, there is shown a tensile curve 310 of a cable incorporating the present invention, and line 320 represents the E module of the cable. Line 320 extends and intersects with the abscissa axis at the intersection point ε0, representing the structural elongation at low tensile stresses of the cable.

In the following tables, there are shown comparison tests between the cable with prior art and the cable incorporating the present invention. The test is conducted according to ISO test method ISO RA-30-203 on Zwick-Z020 test machine. When one test is finished, the test machine can automatically provide the following test results and the tensile curve of the test specimen.

Comparison Test 1:

Prior art: 1×3+5×7×0.15 steel cord;

Present invention: 1×3+5×7×0.15 steel cord with adhesive treatment and PU coating.

Structural E module Elongation at elongation ε0 N/mm² 50N Prior art 0.082% 168072 0.125% Present invention 0.021% 184293 0.060% Improvements   −74% +9.6%   −52% against prior art

The test results show that the present invention not only substantially decreases the structural elongation of the cable by 74%, but also further improves the E module of the cable by 9.6%. These two improvements make a substantial progress on the elongation at certain load, the total elongation at 50N decreased by 52%. Besides, the tensile curves in FIGS. 1 and 3 also illustrate this improvement.

Comparison Test 2:

Prior art 1: 7×3×0.15 steel cord;

Prior art 2: 7×3×0.15 steel cord with PU coating;

Present invention: 7×3×0.15 steel cord with adhesive treatment and PU coating.

Structural E module Elongation at elongation ε0 N/mm² 50N Prior art 1 0.044% 176357 0.119% Prior art 2 0.031% 180437 0.105% Present invention 0.004% 182778 0.077% Improvements   −91% +4%   −35% against prior art 1

The test results show that the present invention not only substantially decreases the structural elongation of the cable by 91%, but also further improves the E module of the cable by 4%. These two improvements make a substantial progress on the elongation at certain load, the total elongation at 50N decreased by 35%. Besides, the tensile curves in FIGS. 1 and 3 also illustrate this improvement.

Compared with prior art, the use of an adhesive on the surface of steel filaments before the application of polymer material further improves the anchorage of steel filaments inside polymer material. The steel filaments of the steel cord are constrained from slipping and turning even there are some void spaces unfilled by polymer material, which further limits the structural elongation of the cable. Besides, the improved anchorage of steel filaments inside polymer material also improves the E module of the cable because there is no slippage or peeling between steel filaments and polymer material.

A further improvement to the present invention is characterized by the thickness of the polymer coating of the cable. A cable with polymer coating of 10 μm only marginally increases the diameter of the cable, which is especially valuable for the cable used as tension member to reinforce synchronous belt. Because the synchronous belt is molded in a semi-open mold where the polymer material is poured into the mold or extruded with a low pressure, the polymer material inside the mold has limited ability to flow between the tension members and to form the final requested shape (toothed, flat, even, . . . ). Therefore, a fine cable with less than 10 μm polymer coating will leave more space for the polymer material to flow inside the mold and to form a flat and even belt.

Another application with this fine cable with less than 10 μm polymer coating is for window elevator system. Because the cable used for window elevator system needs to be clamped by metal nipples at the end of the cable to connect other parts, cables with thick polymer coating can not guarantee a secured connection between the end of the cable and the nipple. The nipple clamps on the polymer coating and the polymer coating transfers the tension to the steel cord inside. Since the tensile strength of the polymer material is quite low compared with that of the steel cord, the connection between cable and nipple breaks at low load, and the load transmission ability of the cable is undermined due to this weak point. When a cable with less than 10 μm polymer coating is used in a window elevator system, the metal nipple clamps directly to the steel cord because of the deformation of thin coating of polymer material. This application secures the connection between cable and nipple and eliminates the weak point for the system. Besides, when the coating thickness is less than 10 μm or even 0 μm, from the outside the cable is virtually the same as a bare cable with the same friction and wear properties. This might be of a big advantage when one would like to substitute a bare cable by such a products in a cable system since there is no need to change the guiding part, cable tubes, etc.

Another improvement with present invention is to use the cable as the subject matter of present invention to build a multi-strand rope for hoisting applications such as elevator ropes. Firstly, the elevator industry is looking for ropes with limited elongation. Since the strands have limited elongation, the rope will have a limited elongation either. Hence, elevator ropes using present invention meet this requirement. Secondly, the elevator industry is looking for ropes that are capable of running on small sheave diameters. The standard elevator uses ropes that respect the generally accepted sheave diameter “d” over rope diameter “D” ratio of 40. When traditional all steel ropes are used in conditions where the d/D ratio is lower than 40, the fatigue life of ropes drops significantly. One of the failure causes of the ropes under these conditions is excessive inter-strand and inter-wire fretting. Although a polymer coating on the rope can reduce the fretting and improve the fatigue life of the rope, a thick polymer coating is needed to secure an endurable polymer sheath and the thick coating increase the diameter of the rope. Present invention solves this dilemma. On one hand, steel strands are coated with polymer to reduce the fretting. On the other hand, adhesive treatment improves the connection between steel and polymer and makes a thin coating possible. Therefore, ropes made of cables of present invention are suitable for hoisting applications. 

1. A cable comprising a steel cord and a polymer material, wherein the structural elongation of said cable is less than 0.025%.
 2. A cable as claimed in claim 1, wherein the E module of said cable is 4% greater than the E module of said steel cord.
 3. A cable as claimed in claim 1, wherein characterized in that the polymer filling rate is more than 70%.
 4. A cable as claimed in claim 3, wherein the polymer filling rate is more than 90%.
 5. A cable as claimed in claim 1, wherein the thickness of the polymer coating is less than 100 μm.
 6. A cable as claimed in claim 5, wherein the thickness of polymer coating is less than 100 μm.
 7. A cable as claimed in claim 1, wherein said polymer material is a thermoplastic polymer.
 8. A cable as claimed in claim 7, wherein said thermoplastic polymer is polyurethane.
 9. A rope comprising at least one cable as claimed in claim
 1. 